The Forecast:

All the big theoretical pieces appeared to be in place for an active 2013 Atlantic Hurricane Season:

The long term average positive phase of the Atlantic Multidecadal Oscillation (AMO), which favors above average water temperatures in the Main Development Region (MDR) of the Atlantic Basin, an area bounded by the Tropical of Cancer and ~8° North Latitude, and from 72.5° West Longitude to the sub–Saharan African coast.

A neutral to weakly–leaning positive El Niño Southern Oscillation, which typically limits wind shear across the MDR, western Caribbean, and Gulf.

A favorable phase of the Madden–Julian Oscillation (MJO), which can provide moisture seeds that can help the growth of tropical cyclones in the MDR, arrived during the peak of the season in September.

The Reality:

Unfortunately for the forecasters, but fortunately for U.S. coastal residents in harm’s way, the season was uneventful.

The Explanation:

Professional forecasters theorize smaller events, with limited to no predictability for their occurrence more than a couple weeks in advance, played a part.

Serendipity may have been the case with Ingrid’s formation location in the Bay of Campeche rather than the south central Gulf, as well as a northern Caribbean sea disturbance in mid August that emerged into the southern Gulf only to be sheared by moderate southwest flow, and killed off by dry atmospheric air that surged into the western Gulf courtesy of the “La Canícula” high pressure ridge.

Deep atmospheric wind shear, a known enemy of tropical cyclone development, dominated good portions of the MDR for much of the peak Cape Verde season (August 1 through September 30).

The second primary enemy of tropical cyclone development, dry air, dominated the MDR and much of the Caribbean and Gulf of Mexico during the summer.

A weak Sub Saharan Jet Stream, suggested by Brian McNoldy of the University of Miami’s Rosenstiel School of Marine and Atmospheric Science, may have reduced the necessary amplitude and curvature of easterly waves emerging off the coast of West Africa.

Anomalous low pre–season phase of the AMO creating a period of low monthly index values of the Thermohaline Circulation(THC)/AMO (Klotzbach and Gray, 2013) from April to June may have contributed to the increased dry air and northerly flow that supported the dry air through the peak of the season (August and September). Though THC/AMO values recovered by July, the lagged influence of the April to June period was likely not enough to overcome the dry and stable airmass.

The pronounced positive phase of the North Atlantic Oscillation (NAO), which dominated much of April and May, perhaps correlated with the low AMO phase in time to counteract warm and humid conditions conducive to storm development.

Unlike the 2010–2012 seasons, which were dominated by negative phase of the NAO and showed distinct recurvature of most storms into the open ocean, the 2013 season had fortuitous timing of alternating positive and negative phases. A pronounced positive phase in July arrived prior to the peak of the Cape Verde/MDR season; the phase turned decidedly negative for most of August, reducing the opportunity for U.S. landfalls. In September, the NAO turned positive again (and rainfall increased across Texas), but the persistent dry air and wind shear across the MDR kept development to a minimum.

The last “major hurricane” to make U.S. landfall was Wilma in 2005. The U.S. has now gone eight years without a “major hurricane” landfall. Since 1878 when relatively reliable landfall data became available, the US has never had an eight-year period without a major hurricane landfall.